1. Field of the Invention
The present invention relates to a process for decomposing a perfluoro-compound. More specifically, the present invention relates to a process for decomposing a perfluoro-compound in the gas phase with water vapor or water vapor and molecular oxygen in the presence of a specific catalyst.
2. Description of the Related Art
Perfluoro-compounds are mainly used in semiconductor etching process and the like, in the form of a fluorine-containing etching or cleaning gas. Also, sulfur hexafluoride, particularly excellent in electrical insulation, is used in cables, condensers, transformers and so on. Perfluoro-compounds are also widely used in other fields since they are generally stable and not toxic to a human body.
Nevertheless, it has been recently pointed out that perfluoro-compounds have a large global warming potential and are one of the causes of global warming. Due to progress of the global environment protection movement, technology for decomposing perfluoro-compounds to non-toxic substances which do not influence the global environment, has been demanded.
Among conventional methods of decomposing perfluoro-compounds, particularly fluorocarbons, burning method where CF.sub.4 is burned over a sufficient time period can be mentioned (WO94/05399). An improvement of a burning apparatus used therefor was also proposed (Japanese Unexamined Patent Publication (Kokai) No. 8-309147). However this apparatus has problems, for example, the treatment capacity is small and the apparatus requires expensive materials. A bombing method was also proposed for decomposing fluorocarbon (Japanese Unexamined Patent Publication (Kokai) No. 6-54925). This method allows complete decomposition of fluorocarbon but continuous mass treatment is difficult due to its form.
The other conventional methods of decomposing fluorocarbons include a cement kiln burning method, a plasma decomposing method, a supercritical water method, etc. However, these method also have problems. The apparatus used in the cement kiln burning method is limited in availability depending on the place and is not a general treatment apparatus. The plasma decomposing method has problems in large apparatus and operating cost. The supercritical water method have problems in installation and operation due to the high temperature and high pressure conditions.
In addition to these conventional methods, methods using a decomposing agent were recently proposed. The methods using a decomposing agent include a method of treating a gas containing fluorocarbon with a treatment agent in which an alkali earth metal salt is supported on activated carbon (Japanese Unexamined Patent Publication (Kokai) No. 61-35849), a method of removing by reacting with a solid hydride (Japanese Unexamined Patent Publication (Kokai) No. 8-257359), a method of treating with a decomposing agent at a high temperature of 1100.degree. C. or higher, optionally in the copresence of hydrogen fluoride (Japanese Unexamined Patent Publication (Kokai) Nos. 7-116466 and 7-132211), and so on. However, these decomposing agents are consumed along with the treatment of fluorocarbon and must be supplemented at least to the theoretical amount, making the methods significantly uneconomical and, therefore, the methods are not suitable for mass treatment.
A catalytic decomposition of fluorocarbons has been investigated to solve the above problems of the above conventional fluorocarbon decomposition methods.
Japanese Examined Patent Publication (Kokoku) No. 6-104183 can be mentioned as a fluorocarbon catalytic decomposition method. By the catalysts disclosed in this publication, chlorine-containing fluorocarbons (C.sub.2 Cl.sub.3 F.sub.3, CC1.sub.2 F.sub.2, CHClF.sub.2, CClF.sub.3, etc. ) are completely decomposed. However, these catalysts do not exhibit a catalytic activity for a perfluorocarbon of CF.sub.4 even at an elevated temperature, demonstrating the difficulty in catalytic decomposition of perfluorocarbons.
The proposed decomposition methods for NF.sub.3 or SF.sub.6 are similar to those for fluorocarbons. The NF.sub.3 decomposition methods reported include decomposition by zeolite in Chem. Mater. (1996), 8(6) 1217-1221, decomposition by alumina or silica in DE4311061 or Japanese Unexamined Patent Publication (Kokai) No. 62-273039, and decomposition by activated carbon supporting magnesium oxide in Japanese Unexamined Patent Publication (Kokai) No. 5-15740. The SF.sub.6 decomposition methods reported include decomposition by cement kiln in Cim., Betons, Platres Chaus (1991), 791, 218-220 (CA; 161:240708) and treatment by a decomposing agent comprising a hydrogen fluoride-treated inorganic oxide in Japanese Unexamined Patent Publication (Kokai) No. 7-116466.
However, these NF.sub.3 or SF.sub.6 decomposition methods also have problems similar to those of the fluorocarbon decomposition methods and a catalyst effective for catalytic decomposition is demanded.
As described above, a catalyst which has a sufficient activity and life for catalytic decomposition of perfluoro-compounds has not been reported. The present inventors considered the above and propose a process for catalytically decomposing a perfluoro-compound, which provides a sufficient catalytic activity of decomposing a perfluoro-compound at a relatively low temperature and with a long catalyst life.